1. Field of the Invention
The present invention relates to a semiconductor device including a bonding pad for use in inputting/outputting electrical signals and a method of manufacturing the semiconductor device.
2. Discussion of the Related Art
When a voltage is applied to aluminum (Al) wirings, Al atoms drift by electromigration (hereinafter, called "EM").
The velocity of the diffusion flow (atomic flux) by EM is represented by the below equation (1).
On the other hand, when Al atoms drift, since a density gradient occurs in the aluminum wiring, a stress gradient occurs which negates the density gradient. The stress gradient acts to prevent drift of Al atoms by EM. Thereby, a so-called back flow effect may occur. The velocity of the diffusion flow (atomic flux) by back flow effect is represented by the below equation (2). EQU JEM=(Da.multidot.N.multidot.j.multidot.e.multidot..rho..multidot.Z)/kT (1) EQU JSG=(Da/k.multidot.T) (.DELTA..delta.(x)/.DELTA.x (2)
Here, Da is a diffusion coefficient, N an atom density, T an absolute temperature, j a current density, e a principle electric charge, .rho. an electric conductivity, Z an effective electric charge of metal, .delta. a stress in the longitudinal direction of the wiring, and k a Boltzmann Constant.
In such situation, when the diffusion flow velocity (atomic flux) induced by EM is well balanced with that induced by the stress gradient, Al atoms do not move at all, although in practice, the flow velocity (atomic flux) may not be zero. However, the value thereof becomes very small. The length of the aluminum wiring at which this balanced effect occurs is called "Blech Length". That is, wiring of a length equal to or shorter than Blech Length will show little or no movement of Al atoms. Wiring of a length longer than Blech Length will show substantially more movement of Al atoms. It is well known that wiring not longer than Blech Length has a very long lifetime, compared to wiring that is longer then Blech Length.
Furthermore, for aluminum wiring having a width and current density in the subhalf-micron generation range, Blech Length can be estimated to be about 20 .mu.m.
Generally, writings of the subhalf-micron generation range are formed with aluminum wirings, and contact holes and via-holes are filled with tungsten (W). When a voltage is applied to the wirings, the Al atoms drift by EM and the diffusion flow velocity (atomic flux) is interrupted at interfaces of the contact holes and the via-holes. Consequently, Al accumulation/depletion occurs at the interfaces, and hillocks and voids may occur, which can result in failure. Regarding the holes themselves, since tungsten (W) has larger EM-resistance than aluminum, W accumulation/depletion does not occur in the holes. Therefore, additional failure does not occur.
In the article, "Permitted electromigration of tungsten-plug vias in chain for test structure with short inter-plug distance" of VMIC 266-272(1994), a method of improving the lifetime of the aluminum wiring is reported. In FIG. 8, a structure capable of improving the lifetime of the aluminum wiring is illustrated. In such structure, the distance between the holes 21 which are filled with W, is not longer than Blech Length. By making the length of the aluminum wiring between holes 21 not longer than Blech Length, the lifetime of the aluminum wiring can be improved.
However, the distance between the bonding pad 22 and the hole 21 nearest to the bonding pad 22 is usually about 50-100 .mu.m. Accordingly, this aluminum wiring typically cannot be made the same or shorter than Blech Length. As a result, in the neighborhood of the hole 21 nearest to the bonding pad 22, the lifetime of the wiring decreases.
The structure of the bonding pad has been proposed in the published specification of Japanese Laid-open Patent Publication No. 60-227444/1985. In the structure, the bonding pad is formed with plural aluminum layers and holes for connecting the plural aluminum layers with each other. However, since the holes formed in the bonding pad are filled with upper aluminum layers of the bonding pad, the lengths of the bonding pad and the bonding wire are regarded as part of the wiring length in the hole nearest to the bonding pad as in the background case mentioned above. Accordingly, the length of the aluminum wiring is typically longer than Blech Length. Consequently, the lifetime of the wiring is decreased in the neighborhood of the hole nearest to the bonding pad.